This invention relates in general to a fast frame recorder which records an event at a fast frame rate and plays back the event at a slower frame rate so that the event may be analyzed. More particularly, this invention relates to a fast frame recorder having a solid state memory which has a variable pre-event/post-event recording capability.
Fast frame recorders are useful for motion analysis of an event. A fast frame recorder records a great number of images during an event at a high or fast image frame rate and reproduces the image frames more slowly at a lower frame rate. Thus, any movement occurring during the event may be analyzed in a step-by-step progression. Applications for a fast frame recorder include, malfunctions in high speed machinery, movements of an athlete, testing of safety equipment, shattering an object, etc. One type of fast frame recorder is disclosed in commonly assigned U.S. Pat. No. 4,496,995, issued Jan. 29, 1985. As disclosed in the latter patent, the fast frame recorder-motion analyzer includes a video camera, a variable speed magnetic tape processor and a video display monitor. The camera is read out in block format so that a plurality of lines of video information that correspond to rows of photosites in the camera solid state imager are simultaneously recorded on magnetic tape in parallel longitudinal tracks. During play back, the magnetic tape is played back at a reduced tape speed. A plurality of parallel video signals reproduced from a plurality of parallel tracks on the tape, are processed into a serial video signal which may be used with standard video monitors. Although a magnetic tape, fast frame recorder is advantageous because of its ability to record a large number of image frames and because of the non-volatility of the image storage, there are limitations to magnetic tape recording. Such a fast frame recorder tends to be costly since, in recording and reproducing a plurality of parallel video signals, separate record and reproduce signal processing circuitry must be provided for each video signal channel. Since the video signals are recorded directly on magnetic tape in an analog format, picture quality is degraded significantly due to induced flutter and other noise, due to reduced bandwidth and increased phase distortion and due to imprecisely recorded pixel signal values.
Flutter is the change in video signal caused by variations of tape speed relative to the record and playback magnetic heads. Flutter displays itself on a video monitor as vertical bands of light and dark superimposed over the video. These bands tend to change intensity and position and appear to flutter about the screen during playback. The degree of flutter caused during playback directly effects the intensity and degree of annoyance of light and dark bands of the video display. The degree of flutter will vary from recorder to recorder because of the sensitivity to mechanical tolerance buildup, tape and transport wear and environment. Large amounts of flutter will be introduced when the magnetic tape transport is subjected to shock or vibration during recording or playback.
Besides the image degradation caused by flutter, there are other types of noise that effect the quality of video in a magnetic tape recording system. Tape media has a graininess associated with it that will limit the dynamic range of a recorded analog signal. This graininess can be associated with hiss or snow as displayed on a monitor. Moreover, imperfections in the tape media known as dropouts will result in gaps of missing video. These gaps can be anywhere from a small segment of an image frame to many image frames in size. In addition, electronic noise is injected into a video signal because of analog recording. This particular type of noise is significant and difficult to reduce because of the low level of analog signals being processed. Electronic noise also appears as snow on a monitor and adds to the overall snow content present. As the snow content becomes larger relative to the picture information, there is a reduction in contrast of the pictures.
Limited bandwidth and phase distortion result from trying to get the highest possible frame record rate given the limitations of the write rate and tape speed. This distortion shows on the monitor as a blurring or smearing of image boundaries that are vertically oriented and is particularly evident at extreme edge transitions of black to white or white to black.
In many applications requiring the use of a fast frame recorder, the event to be recorded is random in nature and may occur at any moment during an extended period of time. Due to the inherent recording length constraints of a magnetic tape fast frame recorder, its ability to capture a randomly occurring event may require the recording of a large number of image frames which are unnecessary for the analysis of the event. During playback, the unwanted images must be scanned before images relating to the event can be analyzed, resulting in a waste of time, money and recording tape.